Polycarbonates are well known as tough, clear, highly impact resistant thermoplastic resins. Polycarbonates, however, possess relatively high melt viscosity. The polycarbonate of 4,4'-isopropylidenediphenol (BPA), for instance, is a well known engineering molding plastic.
In order to prepare a molded article from polycarbonate, relatively high extrusion and molding temperatures are required. In order to reduce the melt viscosity while also maintaining the desired physical properties, methods including the addition of plasticizers, the incorporation of aliphatic chainstoppers, the reduction of molecular weight, and the preparation of blends of polycarbonate with other polymers have been practiced. Known methods also include the addition of diacid residues into the polycarbonate to produce polyestercarbonate.
U.S. Pat. No. 5,510,448 discloses a copolyestercarbonate composition derived from a dihydric phenol, a carbonate precursor, and an aliphatic alpha omega dicarboxylic acid or ester precursor.
U.S. Pat. No. 5,025,081 discloses a process of preparing a coplyestercarbonate in which an aliphatic alpha omega diacid is incorporated into aromatic polycarbonate backbones. The process involved a method in which the pH is adjusted in a stepwise manner.
U.S. Pat. No. 4,983,706 discloses a process for preparing a polyestercarbonate which comprises reacting interfacially a dihydric phenol, a carbonate precursor, and the salt of an aliphatic alpha omega dicarboxylic acid having from 8 to about 20 carbon atoms.
Conventional industrial plants synthesize polycarbonate by mixing together an aqueous solution of dihydric compound (e.g., bisphenol-A) with an organic solvent (e.g., dichloromethane) containing a carbonyl halide (e.g., phosgene). Upon mixing the immiscible organic and aqueous phases, the dihydric compound reacts with the carbonyl halide at the phase interface. Typically, a phase transfer catalyst, such as a tertiary amine, is added to the aqueous phase to enhance this reaction. This synthesis method is commonly known as the "interfacial" synthesis method for preparing polycarbonate.
Typically, the preparation of polycarbonate, for example BPA polycarbonate, by the interfacial method is performed at a pH of from about 9 to about 11, more typically from about 9.5 to about 11. Performing the reaction at the higher pH has the advantages of better pH control, reduced build-up of phosgene in the reactor and no hydrolysis of carbonate salts to produce carbon dioxide and subsequent pressure build-up in the reactor. It would be very advantageous, therefore to be able to perform the interfacial polymerization reaction at the highest pH possible.
In order to obtain incorporation of the diacid into the polycarbonate to produce a polyestercarbonate, however, a pH profile is used. By "pH profile" it is meant that the reaction is conducted at different pHs for different periods of time during the reaction. Typically, the pH of the reaction mixture is maintained at about pH 8 to about 8.5 as long as the diacid is present in the reaction mixture. Then the pH of reaction mixture is raised to about pH 10 to about 11. If the pH is held higher than about 8.5 during the initial part of the reaction, not all of the diacid will be incorporated into the polyestercarbonate. If the pH is not raised to above about pH 10 during the latter period of the reaction, some of the diacid may be incorporated as a thermally unstable anhydride.
It would be desirable to develop a process whereby the pH of the interfacial process for the production of polyestercarbonates could be performed at higher pHs.